Distribution and Abundance of Exotic Earthworms Within a Boreal Forest System in Southcentral Alaska
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A peer-reviewed open-access journal NeoBiota 28: 67–86Distribution (2016) and abundance of exotic earthworms within a boreal forest system... 67 doi: 10.3897/neobiota.28.5503 RESEARCH ARTICLE NeoBiota http://neobiota.pensoft.net Advancing research on alien species and biological invasions Distribution and abundance of exotic earthworms within a boreal forest system in southcentral Alaska Deanna Marie Saltmarsh1, Matthew L. Bowser2, John M. Morton2, Shirley Lang3, Daniel Shain3, Roman Dial1 1 Department of Environmental Science, 4101 University Drive, Alaska Pacific University, Anchorage, AK 99508, USA 2 US Fish and Wildlife Service, Kenai National Wildlife Refuge, Soldotna, AK 99669, USA 3 Biology Department, 315 Penn St., Rutgers The State University of New Jersey, Camden, NJ 08102, USA Corresponding author: Matthew L. Bowser ([email protected]) Academic editor: J. Sun | Received 25 June 2015 | Accepted 21 September 2015 | Published 8 January 2016 Citation: Saltmarsh DM, Bowser ML, Morton JM, Sirley Lang S, Shain D, Dial R (2016) Distribution and abundance of exotic earthworms within a boreal forest system in southcentral Alaska. NeoBiota 28: 67–86. doi: 10.3897/neobiota.28.5503 Abstract Little is known about exotic earthworms (Oligochaeta: Lumbricidae) in Alaska outside its southeastern panhandle. This study documents the distribution of exotic earthworms in the relatively undisturbed Kenai National Wildlife Refuge (KNWR), a large, primarily wilderness refuge in southcentral Alaska. We sampled 69 sites near boat launches, along road corridors, and in low human impact areas > 5 km from the road, finding three species of earthworms (Dendrobaena octaedra, Dendrodrilus rubidus, and Lumbricus ter- restris). Most road sites (90%) and boat launches (80%) contained earthworms; half (50%) of low human impact sites contained earthworms. Distance to roads was the only significant factor in predicting earth- worm occurrence; soil pH, soil moisture, leaf litter depth, and vegetation cover were not. The disparate distributions of these three species suggest that within the KNWR road construction and vehicle traffic played a role in dispersal of the widespread, abundant Dendrobaena octaedra and uncommon Dendrodrilus rubidus; bait abandonment appeared to be the primary method of introduction of Lumbricus terrestris. While the distribution of harmful anecic earthworms in KNWR is currently limited, the prohibition of Lumbricus spp. as bait within conservation units in Alaska may be warranted. Keywords Lumbricidae, earthworm invasion, taiga, bait abandonment, non-native species Copyright Deanna Marie Saltmarsh et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. 68 Deanna Marie Saltmarsh et al. / NeoBiota 28: 67–86 (2016) Introduction Pleistocene glaciations extirpated native earthworms from much of North America, leav- ing landscapes devoid of earthworms until the introduction of exotic earthworms (Oli- gochaeta; Lumbricidae) during European settlement (Hale et al. 2005, 2006, Frelich et al. 2006, Holdsworth et al. 2007a, 2007b). The effects of exotic earthworms on forest ecosystems are well documented (Hale et al. 2005, 2006, Frelich et al. 2006, Holdsworth et al. 2007a, 2007b) and vary by feeding strategy. Leaf litter-dwelling, small-sized epigeic species are least destructive, consuming and mixing the top organic layers into textured, homogeneous litter. Endogeic species burrow through the top soil horizon; their physical effects on ecosystem ecology are greater than epigeic worms but less than anecic worms. Anecic earthworms penetrate deep into the soil, transporting surface litter into the min- eral layer (Addison 2008) and increasing soil porosity and water infiltration (Anderson 1988). Removal of leaf litter and deposition of casts on the soil surface by anecic earth- worms can also increase soil erosion and nutrient run-off (Edwards and Bohlen 1996). Material transport by anecic worms, their large adult size, and dense populations have led to substantial ecosystem changes in some parts of North America (Frelich et al. 2006). Earthworms can accelerate litter decomposition (Hale et al. 2006, Suárez et al. 2006, Holdsworth et al. 2007a, 2007b, Addison 2008) and reduce plant species richness (Hale et al. 2006, Holdsworth et al. 2007a, 2007b). Suárez et al. (2006) found that litter remaining in earthworm-invaded areas in New York was 30-60% less than in reference plots. Holdsworth et al. (2007a) found in a Wisconsin forest that exotic earthworms reduced plant species richness in heavily invaded plots by 17%. Similarly, Hale et al. (2006) documented a negative relationship between exotic earthworm di- versity and plant diversity in a Minnesota hardwood forest. Most studies of exotic earthworms have occurred in temperate regions (Hale et al. 2006, Suárez et al. 2006, Holdsworth et al. 2007a, 2007b, Addison 2008); less is known about the distribution and effects of earthworms in subarctic boreal forests (Cameron et al. 2007, Cameron and Bayne 2009, Sanderson et al. 2012). In northern Alberta, Cameron et al. (2007) found boat launches and roads had the highest prob- ability of earthworm occurrence. Their results suggested vehicle transport and bait abandonment as primary mechanisms of earthworm introduction. As for most invasive species, human activities, particularly road construction and unintentional transport, likely increase the rate of spread for exotic earthworms above their natural dispersal rate of 5-10 meters a year (Gundale et al. 2005, Addison 2008). Consequently, exotic earthworms more likely occur near roads due to availability of habitats disturbed by road construction and maintenance that allow for potential es- tablishment, as well as the creation of dispersal corridors (Cameron et al. 2009). Vehi- cles themselves function as dispersal vectors for earthworm cocoons, which are sticky, mucus coated sacks containing developing embryos (Gundale et al. 2005). Several spe- cies such as Lumbricus terrestris (anecic) and Lumbricus rubellus (epi-endogeic) are sold commercially as fishing bait and are possibly introduced into ecosystems when anglers discard unused bait (Cameron et al. 2007). Distribution and abundance of exotic earthworms within a boreal forest system... 69 Seventeen species of earthworms are known to occur in Alaska (see records in Gates 1972, 1974, Reynolds et al. 1974, Reynolds and Wetzel 2008, Reynolds 1977, 1980, Berman and Marusik 1994, Costello et al. 2011, Rinella et al. 2014, and Suppl. material 1: Alaska earthworm records). Of these, 14 are exotic worms introduced from the Palearctic and have become established. Eisenia fetida (Savigny, 1826), a Palearctic species, is commonly used for indoor vermicomposting in Alaska, but due to its low cold tolerance (Greiner et al. 2011, Meshcheryakova and Berman 2014), it is unlikely to become established in Alaska. Two species of earthworms found in southeast Alaska (Arctiostrotus sp. and Sparganophilus sp.) may be native to Alaska or may have been transported from elsewhere in North America. Factors such as pH and temperature likely limit earthworm distribution, especially in boreal regions like Alaska (Chan and Mead 2003, Addison 2008). Earthworms are usually associated with soil pH of 5-7.4, although D. octaedra inhabits soil pH as low as 2.8-3.6, and L. rubellus has been found in areas with pH ≥ 3.0 (Addison 2008). Survival of earthworms in low temperature areas depends on the species and stage of development (Greiner et al. 2011, Meshcheryakova et al. 2014). Meshcheryakova and Berman (2014), by comparing cold hardiness and present distributions of earthworm species in Siberia, concluded that varying cold tolerance of the species considered con- tributed toward their present distribution ranges. A rapidly warming climate in Alaska is likely improving environmental condi- tions for earthworms. Wetlands in Alaska are warming and drying (Klein et al. 2005, Riordan et al. 2006, Berg et al. 2009) and average winter temperatures have warmed 3.5 °C in the last 50 years (Karl et al. 2009). Drying wetlands and warmer winters may provide increasingly suitable habitat for exotic earthworms. Addison (2008) sug- gested that even small increases in winter temperatures will lead to large increases in earthworm habitat. The present study documents species composition, distribution, and habitat cor- relates for earthworms in the Kenai National Wildlife Refuge, a conservation area in southcentral Alaska. A secondary goal is to examine relationships between earthworm occurrence and distance from human-disturbed areas, such as roads and popular fish- ing areas. The final goal is to identify factors potentially limiting earthworm distribu- tion, such as pH and soil moisture, which are likely to change as the climate continues to warm on the Kenai Peninsula. Methods Study area Located on the Kenai Peninsula in southcentral Alaska, USA (60°N, 150°W), the Ke- nai National Wildlife Refuge (KNWR) covers 777,000 ha. Mountains and glaciers characterize the southeastern KNWR (Figure 1). The Kenai Lowlands, mantled by glacial deposits that vary in texture and are capped by silt loam derived from post- 70 Deanna Marie Saltmarsh et al. / NeoBiota 28: 67–86 (2016) Figure 1. Map of sampling locations and earthworm occurrences by species. glacial windblown loess, cover the western portion of